1. Field of the Invention
This invention relates to a method of producing molten aluminum-killed steel as a raw material for cold-rolled steel sheets or the like. More particularly, this invention relates to a method which prevents clogging of an immersion nozzle by Al.sub.2 O.sub.3 -system inclusions. Such clogging can occur when molten steel is poured from a tundish into a mold through an immersion nozzle. In addition, this invention inhibits the rusting and surface defects on a cold-rolled steel sheet caused by Al.sub.2 O.sub.3 -system inclusions.
2. Description of the Related Art
Recently, with the development of the secondary refining techniques, the formation of a thin steel sheet through continuous casting of aluminum-killed steel has become possible. In particular, the development of vacuum degassing techniques has made such production possible.
The formation of a thin steel sheet through continuously casting of aluminum-killed steel using vacuum degassing techniques typically includes the following four steps:
a. converter steelmaking, PA1 b. a vacuum degassing treatment in a ladle, PA1 c. pouring molten steel from the ladle into a tundish, and PA1 d. continuous casting.
Ordinarily, A is added after tapping from the converter (C.gtoreq.0.02%) or after the vacuum degassing treatment (C&lt;0.02%) to deoxidize molten steel. The additions of Al, however, generate fine clusters of high-melting-point Al.sub.2 O.sub.3 -system inclusions. These high-melting-point Al.sub.2 O.sub.3 -system inclusions cannot be floated and separated by performing vacuum degassing in the ladle. Consequently, the Al.sub.2 O.sub.3 -system inclusions attach to the inner surface of an immersion nozzle to clog the nozzle when the molten steel is poured from the ladle into the tundish.
To reduce such nozzle clogging, a method of blowing an inert gas into the nozzle and a method of adding Ca to convert Al.sub.2 O.sub.3 -system inclusions into a low-melting-point oxide composite material consisting of Ca and Al.sub.2 O.sub.3 are known. However, the method of blowing inert gas into the nozzle entails the risk of inert gas being introduced into the mold which causes surface defects in a casting under certain blowing conditions. Moreover, the technique of adding Ca to prevent attachment of alumina inclusions to the inner surface of an immersion nozzle fails to address the problem of rust formation on a product of casting under various operating conditions.
Methods provided to overcome these problems, e.g., those disclosed in Japanese Patent Laid-Open Nos. 276756/1986 and 599/1994, are known.
In the method disclosed in Japanese Patent Laid-Open Nos. 276756/1986, aluminum-killed steel having a C concentration of 0.015 wt. % or less is prepared and Ca or a Ca alloy is added to the molten steel in the melting step or during continuous casting to provide 2 to 40 ppm residual Ca in the molten steel in an attempt to prevent immersion nozzle clogging and product rusting.
In the method disclosed in Japanese Patent Laid-Open No. 599/1994, immersion nozzle clogging and product rusting is limited by adding Ca to a molten aluminum-killed steel having an ultra low-carbon content. Ca concentration is maintained in the range of 5 to 10 ppm, and the inner surface of the immersion nozzle is formed by a refractory material having a CaO content of 15 wt. % or more.
Each of the above-described methods makes it possible to prevent immersion nozzle clogging, but fails to adequately prevent product rusting because neither method can be adapted to a wide range of operating conditions.
That is, with respect to the above-described methods, controlling the generation of CaS, which is a crucial factor in rusting, has not been considered. For this reason, concurrent prevention of nozzle clogging and rusting has not been adequately achieved.